A MASTER PLAN FOR WET WEATHER FLOW MANAGEMENT DEVELOPED FOR THE CITY OF TORONTO

The City of Toronto has developed a Wet Weather Flow Management Master Plan incorporating a new philosophy in wet weather flow management where rainwater is recognized as a resource; wet weather flows are to be managed on a watershed basis; and a hierarchical approach to wet weather flow management is to be used, starting with at source, followed by conveyance and finally end-of-pipe control measures. Two major sets of alternatives were analyzed to establish whether receiving water and infrastructure protection and renewal objectives could be achieved. Source controls at the lot level [such as low impact development: LID techniques] and conveyance control measures were insufficient. A set of five alternative strategies were formulated. Each of the strategies were developed, in accordance with the adopted hierarchical principle and varied in terms of three levels of targets: status quo, moderate enhanced and significant enhanced, to reflect increasing levels of ecological enhancement and infrastructure renewal. The preferred strategy, aimed at achieving the ambitious goal of meeting Provincial Water Quality Objectives in area surface waters may take 75 to 100 years to implement, at a cost estimated to be in the order of CAN. 12 billion, and required a considerable number of technologies not presently used for remediating stormwater runoff. Consistent with the planning horizon for Master Plans, a 25-year implementation schedule was developed from the preferred strategy. Overall benefits expected in the first 25 years include: swimmable waterfront beaches; control of combined sewer overflows in compliance with legislative requirements; basement flooding protection; protection of the City's infrastructure from stream erosion; restoration of degraded local streams and aquatic habitat and the reduction of algal growth along the waterfront and improved stream water quality in area watercourses. The cost of the Plan over the 25 years is estimated to be CAN 1.047 billion with an additional CAN 233 million in operational and maintenance costs.